It has been known in the past that grains, seeds, and other particulate matter composed of particles of different size and/or density may be classified by projecting such material through a suitable nozzle, allowing such particles to follow different trajectories under the influence of gravity, and collecting those particles following different trajectories in different receptacles (U.S. Pat. Nos. 24,714, 1,358,375, 1,517,509). In some instances centrifugal force has been used to project such particles, but gravity has generally been relied upon as the major force for achieving separation and particularly for discharging the particles from the apparatus once separation has been achieved (U.S. Pat. Nos. 1,517,509, 1,358,375, 1,067,766). The equipment disclosed in such early patents has not been known to have achieved significant commercial recognition or acceptance, and is believed totally unsuitable for continuous high-speed separation of particles suspended in an aqueous slurry such as, for example, finely-divided coal to be desulfurized and concentrated so that it may be more economically transported from one area to another through pipelines.
Certain types of mineral matter in coal are detrimental to health (because of toxic trace and minor elements), to boilers (because of corrosion arising from elements such as sodium, chlorine, etc.), and to the environment (sulfur dioxide emissions, acid rains, etc.). The desirability of removing such impurities prior to use of the coal as a fuel is apparent. To avoid the cost of transporting impurities which must later be removed before use of the coal as a fuel, it is believed highly desirable to extract the impurities prior to transporting the material.
This invention is concerned with an effective, high-speed, continuous method and apparatus for separating finely-divided coal from impurities of greater density and, more generally, to a method and apparatus for centrifugally and continuously fractionating a slurry containing particles of similar size but different densities. The apparatus and method are particularly useful for desulfurizing slurry coal, and removing mineral impurities therefrom, so that such coal may be more economically transported in pipeline systems.
The method and apparatus involve the steps of continuously introducing a stream of an aqueous slurry of finely-divided particles having different densities into an axially-disposed inlet at the lower end of a centrifuge chamber while that chamber is rotating about a vertical axis. The chamber has upper and lower frusto-conical surfaces arranged with their apexes facing each other and meeting along an annular horizontal ridge line. Rotational action is imparted to the stream as it passes through the inlet, and the rotational and inflow rates of the stream are controlled so that a first fraction of slurry particles having a density below a selected density level will follow trajectories leading towards the frusto-conical surface above the ridge or apex line, whereas a second fraction of particles having densities greater than the selected density will follow trajectories leading to the frusto-conical surface below that ridge line. Continuous rotation of the chamber causes the lighter and heavier fractions to travel upwardly and downwardly along the respective frusto-conical surfaces. Such lighter and heavier fractions therefore migrate in opposite directions under the influence of centrifugal force and are continuously withdrawn from the upper and lower ends of the chamber while the centrifuge is in operation.
The selected density for differentiating the particles of the two fractions depends on the material to be treated. In the case of coal, such density may be one selected within the range of specific gravities of about 1.4 to 2.0 and, more suitably, 1.5 to 1.7. For optimum separation, the coal and its impurities should be crushed to a size below about 10 microns in diameter. Efficiency of separation is enhanced if such material is crushed to even smaller sizes of the order of 1 to 5 microns in diameter.
For effective and efficient centrifuge operation, the outlet ports at the centrifuge's upper end should be farther from the vertical axis of centrifugation than the outlet ports at the centrifuge's lower end. The result is a substantial balancing of fluid pressures at both sets of ports under given operating speeds. In addition, the horizontal ridge line which constitutes the common apex for both frusto-conical surfaces should be positioned closer to the lower end of the centrifuge rotor than to the upper end thereof, since it has been found that the density bandwidth of the coal-bearing fraction is substantially greater than that of the pyrite-containing fraction.
In the embodiment disclosed, the means for imparting rotational action to the influent stream takes the form of an axial inlet passage that rotates along with the centrifuge rotor, and particularly a plurality of radially-extending vertically-oriented vanes disposed in the lower portion of the rotor and mounted for rotation therewith. Particles and liquid entering the rotor chamber therefore reach angular velocities essentially the same as that of the rotor prior to contact with the frusto-conical surfaces of that rotor.
Other features, objects, and advantages of the invention will become apparent from the specification and drawings.